Maassen started 2002 by finishing 4th at the 24 Hours of Daytona for Champion Racing. Maassen and Luhr dominated the GT class of the ALMS, winning seven of ten races en route to the championship. These victories included second straight Sebring win, and 3rd career Petit Le Mans win.

In 2004, Maassen acted mainly as a third driver for the longer endurance races, winning at Sebring and Petit Le Mans for Alex Job, and winning the GT class at Le Mans with Petersen Motorsports. 2005 saw him focus mainly on the development of the new Porsche RS Spyder, including racing in the car's debut at Laguna Seca.

In 2006, Maassen would race the RS Spyder and win the LMP2 championship with Lucas Luhr. Maassen and Luhr would drive together for the first six races of the season, including one win (at Miller Motorsports Park) and an overall 2nd place (at Mid-Ohio). Maassen the partnered with Timo Bernhard for the remainder of the season, collecting two wins.

Maassen continued driving for Penske in the ALMS, and competed in the 2008 edition of 24 hour Le Mans with Team Essex where he finished second in the LMP2 class together with John Nielsen and Casper Elgaard.